Experimental and Numerical Investigation of Swirl Induced Self-Excited Instabilities at the Vicinity of an Airblast Nozzle

Abstract

We report on the experimental and numerical investigation of swirl induced self-excited instabilities in the form of precessing helical structures at the vicinity of an Airblast Atomiser. Within the scope of this work, the increase of the knowledge of the fundamental factors governing the precessing vortex core phenomenon (PVC) (Gupta et al. 1984) by applying dual air-flow Airblast nozzles is aimed. This study concentrates on the experimental investigation of the impact of important parameters of a combustor system on the performance characteristics of this instability. In particular, in terms of this work the properties of the PVC are determined by applying two different Airblast Atomisers, one of them producing an attached swirl flame, and another producing a lifted swirl flame. Measurements are also performed for a confined and a non confined flame, aiming to determine the impact of the confining duct on the performance of the PVC. In order to gain some further knowledge, regarding the impact of this aerodynamic instability on combustion of gaseous fuel, the features of PVC are experimentally identified under reacting conditions, by employing a laser light sheet (LLS) measurement technique. By applying the LLS measurement technique, further investigation on the nature of the PVC is also attempted. The power spectral density (PSD) function of the flow field was determined on the basis of raw data provided by 3D Laser Doppler Anemometry (3D-LDA). In order to validate the measurement technique as well as the nature of the instability, the planar Mie-Scattering of the flow was evaluated by employing a high speed camera performing at 12 kHz. The precessing character of the flow was also confirmed by means of a numerical simulation using the 3D Reynolds Stress Model (3D-RSM). The results of the numerical investigation provided some useful information concerning the onset of the instability within the primary swirler as well as its size and amplitude. According to this analysis, a high 512 Flow Turbulence Combust (2009) 83:511-533 frequency instability was confirmed within a region of about one burner diameter downstream of the burner exit. Finally an evaluation of the (LDA) method in terms of providing accurate (PSD) was performed for the case of a swirl flow field.Keywords Precessing vortex core (PVC) · Swirl flows · Airblast Atomiser · Single helix structure · High frequency instability · Self excited aerodynamic instability Nomenclature D Diameter [m] F Frequency [Hz] k Turbulence kinetic energy [m 2 /s 2 ] L Length [m] · n Mean LDA data rate [Hz] R Radius [m] R Autocorrelation function [-] Re Reynolds-no. [-] S Swirl number [-] Sr Strouhal number [-] St Stokes number [-] T Temperature [K] Tu Turbulence [%] U Axial velocity component [m/s] V Radial velocity component [m/s] · V Volume flow [m 3 /h] W Tangential velocity component [m/s] X Radial coordinate [m] Z Axial coordinate [m]Greek Symbols ρ Density [kg/m 3 ] Equivalence ratio [-]

Formation and flame-induced suppression of the precessing vortex core in a swirl combustor: Experiments and linear stability analysis

“…[21]). While the frequencies under reacting conditions are slightly higher than for non-reacting flows, f PVC increases in both cases linearly with Q, which is consistent with previous results from other swirl burners [1,7,11,13,47]. For the last reacting case with P th ¼ 35 kW and / ¼ 0:65, the frequency of 855 Hz is slightly lower than the linear fit.…”

“…In some GT combustors, PVCs are encountered for both non-reacting and reacting conditions [7,[11][12][13]. In other combustors, the PVC forms for non-reacting cases, while it is suppressed for certain, but not necessarily all reacting cases [14][15][16][17][18].…”

Experimental study of vortex-flame interaction in a gas turbine model combustor

“…It can be seen that the frequency increases almost linearly with the air flow rate with a slight decrease towards higher flow rates. The linear increase is typical for PVCs and has been reported in several other studies [37,9,2,36]. Based on the minimum outer diameter of the nozzle (d=25 mm), the corresponding Strouhal number is St=1.35.…”

Influence of air staging on the dynamics of a precessing vortex core in a dual swirl gas turbine model combustor

“…For the reacting case, the frequency of the PVC is between approximately 1000 Hz and 1350 Hz (depending on the air split ratio), or approximately 10% higher than in the non-reacting case. In several swirl flow configurations, precessing vortex cores (PVCs) were observed in the non-reacting and / or reacting flow field under different operating conditions 2,[14][15][16][17][18][19][20][21][35][36][37] . The here examined burner featured a PVC for all examined operating conditions.…”

“…Furthermore, PVCs can interact with thermo-acoustic oscillations 5,13 . In several GT combustors, PVCs were observed both in the reacting and in the non-reacting flow field 6,[14][15][16] , while for other configurations a PVC was present in the non-reacting flow field, but was not present in certain, but not necessarily all, reacting flow fields [17][18][19][20][21][22] .…”